In recent years there has been notable progress and changes in the world of telecommunications. In particular, the development of new integrated electronic devices on semiconductors has contributed to a large number of these changes. Amongst these improvements is the reduction of power consumption in portable electronic equipment. This reduction of energy consumption has permitted a reduction in weight and dimensions of this portable equipment, lengthening at the same time their useful life. All this has resulted in a large decrease in costs, something that has made this type of portable electronic equipment accessible to a large part of the population as for example mobile cellular telephones or portable computers. The effort to reduce the size of electronic circuitry is continuing.
This growing demand for low voltage, low power, low cost and high performance mobile communications equipment has changed the way wireless receivers are designed. A fundamental building block for radio frequency transceivers in such electronic circuitry is a modulator/demodulator. Modulators are used to modulate a data input such as a voice input, onto a carrier frequency. Modulators are also referred to as “mixers” or “multipliers”. Demodulators are used to demodulate data from a high frequency carrier. For example, in a mobile telephone, a modulator/demodulator is used for the telephone transceiver.
Mixers may be broadly classified as either passive or active. Passive mixers, for example, may be based upon a FET or diode ring and provide no conversion gain. However, the active mixer approach yields lower conversion loss and/or gain, and a good degree of balance. A particular type of active modulator that is widely used is the “Gilbert Multiplier Cell”, also referred to as the “Gilbert Modulator”, the “Gilbert Cell” or the “Gilbert Mixer”.
The. Gilbert. Multiplier Cell includes a source-coupled transistor pair, also referred to as the lower transistors or driver transistors, which is coupled to a pair of cross-coupled source-coupled transistor pairs, also referred to as the upper transistors, switch transistors or active mixer transistors. For example, RF inputs may be coupled to the source-coupled transistor pair and a local oscillator is coupled to the pair of cross-coupled source-coupled transistor pairs to produce a demodulated output. The Gilbert Multiplier Cell is extensively described and analyzed in Section 10.3 of the textbook “Analysis and Design of Analog Integrated Circuits” by Paul Gray and Robert Meyer, John Wiley and Sons, N.Y., 1993, pp. 670–675.
Prior mixer topologies exhibit disadvantages as the supply voltage is decreased. In a Gilbert cell mixer, the minimum supply voltage is defined by three transistors connected across the supply voltage as well as series connected the load resistances. The required voltage drop over transistors is technology dependent. Attempts to reduce the supply voltage may result in transistors of the differential or long tailed pairs going into the linear region, which has a number of disadvantageous effects. Also, large current through the switching transistors necessitates a large LO drive to provide the desired IR drop through the switching transistors. Further, higher third-order intercept points are needed to amplify large signals without distortion. However, as technologies shrink so do the supply voltages thereby demanding mixer designs that can operate at lower supply voltages while providing the needed performance.
It can be seen then that there is a need for a mixer that provides lower voltage operation, lower LO drive and better linearity.